Abstract RenderLayer scrollbar creation and destruction so that there

    Abstract RenderLayer scrollbar creation and destruction so that there
        is a clean spot in which to drop in engine scrollbar stuff.

        Reviewed by timo

        * platform/ScrollBar.h:
        (WebCore::ScrollBar::hasPlatformScrollBars):
        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::createScrollbar):
        (WebCore::RenderLayer::destroyScrollbar):
        (WebCore::RenderLayer::setHasHorizontalScrollbar):
        (WebCore::RenderLayer::setHasVerticalScrollbar):
        * rendering/RenderLayer.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@15760 268f45cc-cd09-0410-ab3c-d52691b4dbfc